Supermassive black holes in all galaxies? Maybe not

• Scientists might have been tempted to think that supermassive black holes were at the heart of every galaxy. But that’s not what a new study found.
• Looking at X-ray data from 1,600 galaxies, the researchers found that while most massive galaxies do have central supermassive black holes, only about 30% of smaller galaxies do.
• The result supports the theory that giant black holes are born big from the collapse of enormous gas clouds.

NASA published this original story on December 11, 2025. Edits by EarthSky.

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Supermassive black holes not at the core of all galaxies

Most smaller galaxies may not have supermassive black holes in their centers. That’s according to a recent study using NASA’s Chandra X-ray Observatory. This contrasts with the common idea that nearly every galaxy has one of these giant black holes within their cores.

A team of astronomers used data from over 1,600 galaxies collected in more than two decades of the Chandra mission. The researchers looked at galaxies ranging in heft from more than 10 times the mass of the Milky Way down to dwarf galaxies, which have stellar masses less than a few percent of that of our home galaxy. The Astrophysical Journal published the peer-reviewed results on October 6, 2025. NASA announced the findings in a press release on December 11, 2025.

The team has reported that only about 30% of dwarf galaxies likely contain supermassive black holes. Fan Zou of the University of Michigan in Ann Arbor, led the study. Zou said:

It’s important to get an accurate black hole head count in these smaller galaxies. It’s more than just bookkeeping. Our study gives clues about how supermassive black holes are born. It also provides crucial hints about how often black hole signatures in dwarf galaxies can be found with new or future telescopes.

How can we see black holes to study them?

As material falls onto black holes, friction heats it and produces X-rays. Many of the massive galaxies in the study contain bright X-ray sources in their centers. And that’s a clear signature of supermassive black holes. The team concluded that more than 90% of massive galaxies – including those with the mass of the Milky Way – contain supermassive black holes.

However, smaller galaxies in the study usually did not have these unambiguous black hole signals. Galaxies with masses less than 3 billion suns – about the mass of the Large Magellanic Cloud, a neighbor to the Milky Way – usually do not contain bright X-ray sources in their centers.

The researchers considered two possible explanations for this lack of X-ray sources. The first is that the fraction of galaxies containing massive black holes is much lower for these less-massive galaxies. The second is the amount of X-rays produced by matter falling onto these black holes is so faint that Chandra cannot detect it. Co-author Elena Gallo from the University of Michigan said:

We think, based on our analysis of the Chandra data, that there really are fewer black holes in these smaller galaxies than in their larger counterparts.

Determining which solution is correct

To reach their conclusion, Zou and his colleagues considered both possibilities for the lack of X-ray sources in small galaxies in their large Chandra sample. The amount of gas falling onto a black hole determines how bright or faint they are in X-rays. Scientists expect smaller black holes to pull in less gas than larger black holes. So they should be fainter in X-rays and often not detectable. The researchers confirmed this expectation.

However, they found an additional deficit of X-ray sources in less massive galaxies beyond the expected decline from decreases in the amount of gas falling inward. But scientists can account for this additional deficit if many of the low-mass galaxies simply don’t have any black holes at their centers. The team’s conclusion was that the drop in X-ray detections in lower-mass galaxies reflects a true decrease in the number of black holes located in these galaxies.

Implications for how supermassive black holes form

This result could have important implications for understanding how supermassive black holes form. There are currently two main theories. One is that they grow from smaller black holes, created when giant stars run out of fuel and collapse. The second idea is that the giant black holes are born big from the collapse of enormous gas clouds, so that they have the mass of thousands of suns to begin with. The team’s findings suggest the latter is more likely.

Co-author Anil Seth of the University of Utah said:

The formation of big black holes is expected to be rarer, in the sense that it occurs preferentially in the most massive galaxies being formed, so that would explain why we don’t find black holes in all the smaller galaxies.

This study supports the theory where giant black holes are born already weighing several thousand times the sun’s mass. If the other idea were true, the researchers said they would have expected smaller galaxies to likely have the same fraction of black holes as larger ones.

This result also could have important implications for the rates of black hole mergers from the collisions of dwarf galaxies. A much lower number of black holes would result in fewer sources of gravitational waves to be detected in the future by the Laser Interferometer Space Antenna. The number of black holes tearing stars apart in dwarf galaxies will also be smaller.

Bottom line: A new study looked at 1,600 galaxies and found that the cores of small galaxies are less likely to be home to supermassive black holes.

Source: Central Massive Black Holes Are Not Ubiquitous in Local Low-Mass Galaxies

Via NASA

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